1. Field of the Disclosure
The present invention relates generally to controlling a power converter. More specifically, examples of the present invention are related to controlling switch mode power converters.
2. Background
Electronic devices (such as cell phones, tablets, laptops, etc.) use power to operate. Switched mode power converters are commonly used due to their high efficiency, small size, and low weight to power many of today's electronics. Conventional wall sockets provide a high voltage alternating current. In a switching power converter, a high voltage alternating current (ac) input is converted to provide a well-regulated direct current (dc) output through an energy transfer element to a load. In operation, a switch is turned ON and OFF to provide the desired output by varying the duty cycle (typically the ratio of the on time of the switch to the total switching period), varying the switching frequency, or varying the number of on/off pulses per unit time of the switch in a switched mode power converter.
In general, switching the power switch of the switching power converter to provide the desired output results in switching losses due to turning the power switch ON and OFF. For instance, turn ON loss may occur in the power switch when the voltage across the power switch having a drain capacitance (e.g., the effective capacitance between the drain and source of the power switch), is non-zero when the power switch is turned ON. The energy stored (and dissipated) by the drain capacitance is proportional to the value of the drain capacitance and the square of the voltage across the drain capacitance. The non-zero voltage across the power switch may cause a spike in a switch current through the power switch due to the drain capacitance. The power dissipation during the turn ON may be reduced by decreasing the value of the drain capacitance.
Turn OFF loss in the power switch may occur due to the cross over time for the switch current to fall to zero and the switch voltage across the power switch to increase from zero. The speed at which the switch voltage increases from zero is partially determined by the value of the drain capacitance. The lower the drain capacitance, the faster the switch voltage increases from zero. However, the faster the switch voltage increases from zero, the greater the power dissipation during turn OFF because the turn OFF loss is a product of the instantaneous voltage and current during the crossover time. Thus, the turn OFF loss is also sometimes referred to as crossover loss. Power dissipation during turn OFF may be reduced by increasing the value of the drain capacitance such that the switch current has substantially fallen to zero before the switch voltage increases from zero which minimizes the crossover time and therefore the turn OFF loss. Consequently, there have been compromises between reducing turn ON losses and reducing turn OFF losses of the power switch.
Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.